1. Field of the Invention
The present invention relates to a fuel cell system that distributes and supplies fuel gas to a plurality of power generation cells, and more particularly to a structure that effectively expels unnecessary air as well as liquid that accumulated in a fuel diffusion space of a power generation cell from the power generation cell.
2. Description of the Related Art
In a fuel cell, the amount of electric energy that can be extracted per volume and per weight is significantly greater than for a conventional secondary battery, and the cell body can be used repeatedly by supplying fuel. Consequently, there are great hopes for the use of fuel cells as not only stationary-type power generating equipment, but also as power sources for portable electronic devices such as digital cameras, mobile phones and notebook personal computer.
Since a small and lightweight fuel cell system is suitable as a power supply for a portable electronic device, an air breathing type fuel cell system has been proposed in which atmospheric oxygen and hydrogen gas are reacted using a polymer electrolyte membrane. In the air breathing fuel cell system, in a power generation cell, catalyst layers are disposed on both sides of a polymer electrolyte membrane, with a fuel diffusion layer being overlaid on the catalyst layer on the fuel supply side and an oxygen diffusion layer being overlaid on the catalyst layer on the oxygen supply side. Generally, the air breathing fuel cell system is designed in a form having no recirculation mechanism that causes fuel gas to continuously flow and recirculate in the power generation cell. More specifically, it is a so-called dead-ended fuel cell system in which hydrogen gas supplied from a fuel tank or fuel forming apparatus is fed into a sealed fuel diffusion space and completely consumed with a polymer electrolyte membrane.
However, when a dead-ended fuel supply is performed, impurity gas such as moisture, nitrogen gas, or carbon dioxide gradually increases in the sealed fuel diffusion space. The impurity gas that resides inside the fuel diffusion space lower the partial pressure of the hydrogen gas and obstruct the hydrogen supply to the polymer electrolyte membrane.
U.S. Pat. No. 6,423,437 discloses an air breathing fuel cell system that performs a dead-ended fuel supply. In this case, an absorber of impurity gas is disposed in a power generation cell that is most distant from the fuel supply side of stacked cells, thereby removing impurity gas from the sealed hydrogen gas supply space.
Japanese Patent Application Laid-Open No. H03-205763 discloses a fuel cell system in which hydrogen gas is caused to flow and recirculate. In this system, impurity gas is expelled from the fuel space of a power generation cell by allowing hydrogen gas to flow through the fuel space. Since impurity gas that remains in the fuel space is diluted by newly supplied hydrogen gas, there is no concern of residence/accumulation of impurity gas in the fuel space leading to an increase in the concentration of impurity gas therein.
Japanese Patent Application Laid-Open No. H06-267564 discloses a fuel cell system in which, as a polymer electrolyte membrane, a plurality of power generation cells that use a fluororesin ion-exchange membrane containing sulfonic acid groups are stacked and connected in series. In this system, the thicknesses of oxygen diffusion layers are varied to reduce a variation in the oxygen supply to the polymer electrolyte membrane surface.
According to the passive method of removing impurity gas disclosed in U.S. Pat. No. 6,423,437 above, since impurity gas accumulates in the sealed fuel space, the total amount of impurity gas that can be eliminated is limited. Further, in a case where a fuel cell is left as it is without supplying fuel while a load is connected to the fuel cell, there is a possibility that the hydrogen gas in the fuel space will be completely consumed and the fuel diffusion layer will be completely filled with impurity gas, and in that case it is also difficult to completely remove the excessive impurity gas.
According to the active method of removing impurity gas as disclosed in Japanese Patent Application Laid-Open No. H03-205763 above, although residence/accumulation of impurity gas in a power generation cell can be temporarily avoided, unless an equipment that removes impurity gas is provided in the hydrogen gas circulation system, the impurity gas in the circulation system cannot be fundamentally removed. Further, since provision of a hydrogen gas circulation system itself inhibits size reduction, weight reduction, and cost reduction of the fuel cell system, it is not suitable for a power source of a portable electronic device.
Consequently, technology has been proposed that expels impurity gas which resides/accumulates in a power generation cell automatically or manually when needed by using a purge mechanism for replacing the air in the hydrogen supply system with hydrogen gas. However, unlike the case of a factory, households and the like have no choice other than to utilize a small-capacity fuel tank or a low-pressure forming apparatus, and thus an adequate hydrogen gas pressure or flow rate cannot be secured. Accordingly, in a conventional purge mechanism configured on the assumption of use of a large volume of hydrogen gas at a high pressure, a sufficient purge effect cannot be achieved.